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Starting a car with two weak 12V batteries, possible?

Discussion in 'Electronic Basics' started by Indian Summer, Dec 6, 2003.

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  1. Is it possible to power and automobile starter using 24V on a 12V system,
    being careful not to let the 24V bleed into the rest of the car's circuit? I
    need a system to start my car in case it runs low on battery. I've been
    stranded once and need a way to start the car with just a push of a button
    to join two batteries to make the starter crank.

  2. While it is certainly possible, it may not be practical. It is much
    easier to set up a dual 12 volt battery system that keeps both
    batteries charged while driving, but reserves one of the batteries as
    a back up for the starter, so that leaving the headlights on or
    whatever does not dump both batteries. You just connect both in
    parallel with a contactor, when you hit the starter.
  3. Gareth

    Gareth Guest

    Yes, it is possible, in fact it is common for breakdown recovery people
    to use a 24V battery to start vehicles with 12V electronics. Note that
    it is probably easier to connect your two weak batteries in parallel,
    this will still give you 12V but the internal resistance of the two
    batteries will be lower than either one on its own, so should give more
    current to the starter motor. John Popelish's suggestion of keeping a
    reserve battery charged is probably better than connecting two weak
    batteries in parallel.

    The current draw by a car starter motor is very high so make sure that
    the wires and connectors you use can handle the current. You should
    also keep the high current wires short as the resistance (and therefore
    voltage drop) along the wire is proportional to length.

    Do you now why your car let you down? If there is a problem with a
    specific part, such as the battery or alternator, then I would recommend
    that you replace that part rather than mess about with two half worn out

    I hope that helps,


  4. Is it possible to power and automobile starter using 24V on a 12V
    What a great practical idea! So now what kind of diodes ratings should be
    used to controll currents flowing only in one way so that it charges but not
    drain while driving, except for starting?

  5. Is it possible to power and automobile starter using 24V on a 12V
    I'd wanted a battery in the trunk since I sometimes forget to turn off the
    headlights when exiting or run my portable DC electronics too long. Putting
    in the trunk means running a long wire, I have no choice. Let's see what
    happens. And thanks, your idea helped.
  6. Don't forget to make that a "Long HEAVY wire" :)
  7. Doug

    Doug Guest

    I'd just get on of those jumper boxes and through it in the trunk. Yes
    you'll have to get out and jump the battery but its the simplest/ cheapest
    solution. One of the problems with trying to put a battery in the trunk is
    the length of the wire requires a large wire that's hard to handle and
    route. There is also the problem of firmly fastening it to the vehicle so
    it doesn't become a projectile in the event of an acident or you tip it
    over and leaking acid eats the trunk (I've done that, don't ask how I was
    17 at the time). The other factor is that repeatly running a car battery
    down does them no good.
  8. It would make more sense to spend your time and money on prevention circuits
    and fixing whatever is wrong. I'm 63 yrs old and have had many cars and
    problems and many times dreamed of solutions like yours but never found a
    practical one, Headlight reminders, timers and other things are much more
    practical. Getting two 12 volt batteries in series to start and in parallel
    to charge is a big challenge. Depending on how your car is wired you could
    zap more than the starter with 24 volts. Adding another alternator to two
    batteries in parallel would also be a challenge. Most of the world thrives
    on one battery and you can too. Don't waste your time and money. Just my 2
    cents worth.
    hank wd5jfr
  9. Don Kelly

    Don Kelly Guest

  10. Ian Stirling

    Ian Stirling Guest

    A much simpler idea might be to add a loud warning buzzer that goes off
    at 11V or so.
  11. The RV supply house will probably have switches and relays designed
    for this service.

    Another possibility is a starter solenoid, the type that mounts near
    the battery instead of on the starter motor. This is just a
    heavy-duty relay. Connect the main terminals between your spare
    battery and the original battery, bypassing the isolator diode.
    Energizing the solenoid gives the equivalent of a jump start. You
    don't have to worry about disconnecting the main battery, but make
    sure that you can't leave the solenoid energized when you drive away.
    Not only are those relays designed for intermittant duty, but it would
    defeat the purpose of the battery isolator. A push-button or
    spring-return toggle switch will do.

    Remember that the starter motor draws around 100 amps, so use at least
    6-guage wire, and don't depend on the car-body ground. Run the
    negative lead to the same terminal (usually on the engine block) as
    the original battery ground cable.
  12. James Beck

    James Beck Guest

    Here is one ready made :

    Look under battery accessories
  13. als

    als Guest

    Lots of options.
    I have used, in the past,
    1) trunk-mounted main battery
    use the biggest cable you can get into a channel. Welding cable is great.
    2) front or trunk-mounted auxillary battery
    a) isolated from starter; connected only to added loads (transmitter,etc).
    Needs a diode isolator big enough to accommodate max charge.
    b) starter helper - use an old Ford-type solenoid, switch as necessary.
    c) get a (pickup) diesel kit. Available isolated/non-isolated.
    3) 24v alternator/24v system
    a) cross-connected series/parallel operation, or
    b) center grounded, using -12v to power loads

    For the trunk-mounts, use a marine plastic housing, and an old seat belt
    or other suitable hold-down. Used this in destruction derbies. If you
    get hit harder than that, you got other problems. :)
    As already posted, RV kits are available at your local parts store.
    For the 24v method, either get a 24v regulator for the existing alternator,
    or get a 24v alternator/regulator from an old truck. The (12v) alternator
    windings and diodes are (and have been) fine for use at 24v. Got one
    running at 48v, but I would not recommend that for your average bear :)

    <als> als_AT_hal_DASH_pc_DOT_org
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